Multicellular DC/DC voltage converter with protection switches

ABSTRACT

A DC/DC voltage converter between a high-voltage electrical network and a low-voltage electrical network comprises a plurality of cells connected in parallel. Each cell comprises a chopper DC/DC converter and a single protection transistor connected in a high-voltage portion of the converter thereby enabling any of the cells to be taken out of service independently of the other cells while minimizing power consumption in normal operation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of Application PCT/FR03/00209,filed Jan. 22, 2003, which claims priority to French patent application02 00750 filed Jan. 22, 2002 and French patent application FR 02 00751filed Jan. 22, 2002, the disclosures of all three being incorporatedherein by reference in their entirety.

FIELD

The present application relates to a voltage converter and findsapplications, in particular, in the automotive field.

The application relates more particularly to a direct current/directcurrent (DC/DC) voltage converter that is multicellular, i.e. comprisinga plurality of cells forming a corresponding number of respectiveindividual converters connected in parallel with one another. Inparticular, each cell may be a chopper DC/DC converter which may presentthe particular feature of being non-isolated.

BACKGROUND

Such a chopper converter may be a controlled two-port electrical circuitcomprising a first pair of positive and negative terminals and a secondpair of positive and negative terminals. The first and second negativeterminals may be connected together by a first determined circuitbranch. Similarly, the first and second positive terminals may beconnected together by a second determined circuit branch which includesan inductor forming an energy reservoir. The converter may furthercomprise chopper means comprising at least one controlled switch whichis switched OFF and ON with a determined duty ratio under the control ofa management unit.

Such a circuit may be capable of delivering direct and/or quasi-directelectric current between the first pair of positive and negativeterminals at a determined voltage, referred to as the “output” voltage,when a determined voltage, referred to as the “input” voltage, isapplied between the second pair of positive and negative terminals, orvice versa.

The converter is said to be non-isolated in the sense that it comprisesthe first and second circuit branches respectively interconnecting thefirst and second negative terminals and the first and second positiveterminals. Such a converter is contrasted with an isolated converter inwhich the first pair of terminals is isolated from the second pair ofterminals.

In order to reduce the size of the components making up the converter,while delivering sufficient power to feed various items of equipment, itis known, in particular from document U.S. Pat. No. 6,275,958, toimplement a multicellular converter comprising a series of cellsconnected in parallel. When a cell is faulty, it is also known from thatdocument to isolate the faulty cell by means of two protection switchesformed by metal oxide semiconductor (MOS) transistors disposed one on ahigh-voltage network side and the other on a low-voltage network side.

Those transistors operate as controlled switches which are ON in normaloperation and which are OFF when malfunction is detected.

It follows that in normal operation, the components that are dedicatedto the protection function give rise to static consumption of powerwhich, depending on circumstances, can lie in the range 0.5% to 2.0% ofthe static consumption of the circuit as a whole.

Furthermore, the presence of those protection components leads to anincrease in the size of the electrical circuit, to a lengthening andgreater complexity in the manufacturing method, and finally to asignificant increase in the cost of the circuit.

SUMMARY

According to some embodiments of a multicellular voltage converter has afault mode of operation in which a determined cell can be taken out ofservice independently of the other cells when the cell suffers amalfunction, while minimizing power consumption in normal operation.

One embodiment proposes a DC/DC voltage converter comprising:

-   -   a first positive terminal and a first negative terminal for        connection respectively to two terminals of a high-voltage        electrical network;    -   a second positive terminal and a second negative terminal for        connection respectively to two terminals of a low-voltage        electrical network; and    -   n cells connected in parallel, where n is an integer greater        than unity, disposed between said first positive and negative        terminals and between said second positive and negative        terminals, each cell comprising a chopper DC/DC converter, each        having a first circuit branch interconnecting said first and        second negative terminals, a second circuit branch including an        inductor and interconnecting said first and second positive        terminals, chopper means comprising at least one chopper switch,        and a management unit adapted to control OFF and ON switching of        the chopper switch with a determined duty ratio;

in which each cell further comprises a single protection transistordisposed in said second circuit branch and associated with a protectionmanagement unit for taking said cell out of service independently of theother cells.

Contrary to the conventional wisdom of document U.S. Pat. No. 6,275,958,only one protection transistor suffices to enable the corresponding cellto be isolated, and the cell is put into operation by controlling thissingle transistor in order to switch it ON, so that consumption issmaller.

In one embodiment, the single protection transistor of each cell isconnected in the high-voltage portion of the converter. Because of thehigh voltage, the holding current when the transistor is in the ONposition b may be smaller, so consumption may be further minimized.

n cells connected in parallel, where n is an integer greater than unity,disposed between said first positive and negative terminals and betweenthe second positive and negative terminals, each cell comprising achopper DC/DC converter, each having a first circuit branchinterconnecting the first and second negative terminals, a secondcircuit branch including an inductor and interconnecting said first andsecond positive terminals, chopper means comprising at least one chopperswitch, and a management unit adapted to control OFF and ON switching ofthe chopper switch with a determined duty ratio;

in which each cell further comprises a single protection transistordisposed in the second circuit branch and associated with a protectionmanagement unit for taking said cell out of service independently of theother cells.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages appear from the following description ofnon-limiting embodiments given with reference to the accompanyingdrawings, in which:

FIG. 1 is a circuit diagram of an embodiment of a voltage-lowering(buck) converter;

FIG. 2 shows the arrangement of a controller and a malfunction detectorassociated with the FIG. 1 converter;

FIG. 3 is a circuit diagram of an embodiment of a voltage-raising(boost) converter;

FIG. 4 is a circuit diagram of a first embodiment of a buck/boostconverter;

FIG. 5 shows the arrangement of a controller and a malfunction detectorassociated with the FIG. 4 converter;

FIG. 6 is a circuit diagram of a second embodiment of a buck/boostconverter;

FIG. 7 shows the arrangement of a controller and a malfunction detectorassociated with the FIG. 6 converter; and

FIG. 8 is a circuit diagram of a circuit including common protection onthe low-voltage side.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the figures, in order to show clearly the orientation of thetransistors used, the letters D and S designate respectively the drainand the source of each transistor, in correspondence with theorientation specified in the description for that transistor.

In the diagram of FIG. 1, a high-voltage DC network is connected to abuck converter via the positive terminal 1 and the negative terminal 2.The voltage between the terminals 1 and 2 is situated around 42 volts(V), for example. A DC network of low-voltage is connected to terminals3 and 4, the terminal 3 being positive and the terminal 4 beingnegative, and the voltage between these two terminals is situated, forexample, around 14 V.

In the example shown in FIG. 1, six mutually identical individualchopper converters 100, 200, . . . , 600, of the buck type are disposedin parallel between on one side the terminals 1 and 2 and on the otherside the terminals 3 and 4. The terminals 2 and 4 are directlyinterconnected.

In each of these individual converters 100, 200, . . . , 600, acapacitor 16, 26, . . . , 66, e.g. a 30 microfarad (.mu.F) capacitor,interconnects the tenninals 1 and 2 in order to store on the 42 Vnetwork side the electric charge that is transferred by the individualconverter 100, 200, . . . , 600. Similarly, in each individual converter100, 200, . . . , 600, another capacitor 17, 27, . . . , 57, e.g.likewise a 30 .mu.f capacitor, interconnects the terminals 3 and 4 inorder to store, on the 14 V network side, the electric chargetransferred by the individual converter 100, 200, . . . , 600. Inoperation, each of the two networks respectively at 42 V and at 14 V,consumes some of the electric charge in the respective sets ofcapacitors 16, 26, . . . , 66 and 17, 27, . . . , 67.

In each individual converter 100, 200, . . . , 600, the terminals 1 and3 are interconnected by a branch containing the following componentsconnected in series in the following order going from terminal 1 toterminal 3:

-   -   a buck transistor 11, 21, . . . , 61, e.g. a MOS field effect        transistor (MOS-FET) having an n channel (N-MOS), connected by        its drain to the terminal 1. Such a transistor includes an        intrinsic diode 18, 28, . . . , 68 connected in parallel with        the switch constituted by the buck transistor 11, 21, . . . , 61        and oriented to pass current from the terminal 3 towards the        terminal 1;    -   an inductor 14, 24, . . . , 64, e.g. a 12 microhenry (μH)        inductor with a resistance of 6 milliohms (mΩ) connected to the        source of the buck transistor 11, 21, . . . , 61 via a node N1,        N2, . . . , N6;    -   a protection transistor 13, 23, . . . , 63, there being a single        protection transistor for each cell, e.g. likewise an n-channel        MOS-FET. This transistor 13, 23, . . . , 63 is connected via its        drain to the inductor 14, 24, . . . , 64. As for the buck        transistor 11, 21, . . . , 61, this protection transistor 13,        23, . . . , 63 includes an intrinsic diode 20, 30, . . . , 70        connected in parallel with the protection transistor 13, 23, . .        . , 63 and oriented to pass current from the terminal 3 towards        the terminal 1; and    -   a resistor 15, 25, . . . , 65, e.g. a 2 mΩ, connected to the        source of the transistor 13, 23, . . . , 63 and to the terminal        3.

In the event of an accidental short circuit between the source and thedrain of the buck transistor 11, 21, . . . , 61, the equivalent diode20, 30, . . . , 70 of the protection transistor 13, 23, . . . , 63prevents unwanted current flow from the terminal 1 to the terminal 3 solong as the protection transistor 13, 23, . . . , 63 is controlled atthat time to be OFF. Such an involuntary current would lead to thehigh-voltage circuit discharging into the low-voltage circuit.

Each individual buck converter 100, 200, . . . , 600 also includes adiode 12 a, 22 a, . . . , 62 a having its cathode connected to the nodeN1, N2, . . . , N6 and its anode connected to the terminals 2 and 4.

Each of the individual buck converters 100, 200, . . . , 600 asconstituted in this way is capable of transferring approximately 250watts (W) of power from the 42 V network to the 14 V network.

FIG. 2 shows the architecture for controlling the individual converter100 of FIG. 1. It comprises a controller C1 whose inputs receive thehigh voltage by wires 101 and 102 connected respectively to theterminals 1 and 2, said low voltage by wire 103 connected to theterminal 3, and the voltage across the terminals of the resistor 15 bywires 150 and 151. In a control mode using pulse width modulation andknown to the person skilled in the art, the controller C1 controls thebuck transistor 11 by OFF or ON signals transmitted to its grid by wire110, with pulses being at a determined periodicity, e.g. correspondingto a frequency of 70 kilohertz (kHz).

A controller analogous to the controller C1 is connected in the samemanner in each of the five other individual converters 200, . . . , 600to perform an identical function in each of those circuits.Advantageously, the six controllers issue respective pulses at the samepulse periodicity, and are taken into consideration in a determinedcyclical order, so that pulses from two successive individual convertersin the order are offset by a shift equal to one-sixth of the period ofthe control pulses from each individual converter.

For the individual converter 100, a detector D1 has two inputs receivingthe voltage between the drain and the source of the buck transistor 11via, two wires 111 and 112. In normal operation of the individualconverter 100, the detector D1 transmits a signal to a protectionmanagement unit P so that it applies a certain voltage via wire 130 tothe grid of the protection transistor 13, e.g. a voltage lying in therange 5 V to 10 V relative to the source of the protection transistor13, so as to hold the protection transistor 13 in an ON or conductivestate.

When the detector D1 identifies a malfunction of the buck transistor 11,an(d in particular a short circuit between the drain and the source ofthe buck transistor 11, the detector D1 interrupts the voltage appliedto the grid of the protection transistor 13 via the wire 130 so as toopen (switch OFF) the circuit between, the drain and the source of theprotection transistor 13. This circuit opening can be obtained by meansof a bias resistor (not shown and having a resistance of 10 kilohms(kΩ)) connecting together the grid and the source of the protectiontransistor 13. Thus, the entire individual converter 100 is taken out ofservice. In addition, any current discharged from the 42 V network tothe 14 V network via the individual converter 104 cannot flow from theterminal 1 to the terminal 3.

The other individual converters 200, . . . , 600 also have respectivedetectors D2, . . . , D6 identical to the detector D1 and connected inanalogous manner to the protection management unit P. This unit is alsoconnected by wires 230, . . . , 630 to the respective buck transistors23, . . . , 63 of the individual converters 200, . . . , 600 so as toprovide an identical individual protection mechanism to all of theindividual converters 100, 200, . . . , 600.

Since each individual converter 100, 200, . . . , 600 is connected inparallel with all the other individual converters, any one of themceasing to operate does not interrupt the operation of the others.Overall converter operation thus continues by means of the individualconverters that are still operational. This continued operation is madepossible by the individual converters being connected in parallel, andby a protection switch being placed in each individual converter.

Optionally, the control mode for all of the individual converters 100,200, . . . , 600 via their respective controllers can be adapted bymeans of a supervisor controller. (not shown) connected to the sixcontrollers and to the six detectors so as to take account of one of theindividual converters being taken out of service. Under suchcircumstances, this makes it possible to optimize the operation of theoverall converter in spite of one of its individual converters beingtaken out of service.

For example, when one of the individual converters 100, 200, . . . , 600is taken out of service, the supervisor controller controls thecontrollers of the five other individual converters 100, 200, . . . ,600 that are still operational so that the control pulses of twosuccessive individual converters are offset by a period equal toone-fifth of the common period of the control pulses in each individualconverter. The fault mode of operation as obtained in this way for theoverall converter, after one of the individual converters has been takenout of service, corresponds to a reduction in the maximum rate at whichelectric charge can be transferred, or to a reduction in the maximumelectrical power that can be transferred between the high-voltagenetwork and the low-voltage network.

FIG. 3 corresponds to an embodiment which consists in a boost converter.This embodiment repeats the architecture and some of the components ofthe embodiment described above. A detailed description is not repeatedin full, and all components and references that are not repeated areidentical to those described in the above embodiment.

In this embodiment, the single protection transistor 12, 23, . . . , 63is connected in the high-voltage portion of the cell.

Each individual converter 100, 200, . . . , 600 is now a boost convertercomprising:

-   -   a transistor 13, 23, . . . , 63, e.g. an n-channel MOS-FET,        performing the function of a protection switch, having its drain        connected to the terminal 1. It includes an intrinsic diode 20,        30, . . . , 70 connected in parallel with the protection        transistor 13, 23, . . . , 63 and oriented to pass current        flowing towards the terminal 1. Its source is also connected to        the capacitor 16, 26, . . . , 66;    -   a diode 11 a, 21 a, . . . , 61 a having its cathode connected to        the source of the protection transistor 13, 23, . . . , 63;    -   an inductor 14, 24, . . . , 64, e.g. a 12 μH and 6 mΩ inductor,        connected to the anode of the diode 11 a, 21 a, . . . , 61 a via        a node N1, N2, . . . , N6; and    -   a resistor 15, 25, . . . , 65, e.g. a 2 mΩ resistor connected to        the inductor 14, 24, . . . , 64 and to the terminal 3.

Instead of the diodes 12 a, 22 a, . . . , 62 a, each individualconverter 100, 200, . . . , 600 includes a boost transistor 12, 22, . .. , 62. The boost transistor 12, 22, . . . , 62, e.g. still an n-channelMOS-FET, has its drain connected to the node N1, N2, . . . , N6 and itssource connected to the terminals 2 and 4. The boost transistor 12, 22,. . . , 62 also includes an intrinsic diode 19, 29, . . . , 69 connectedin parallel and oriented to pass current towards the node N1, N2, . . ., N6.

The mode of operation of such a boost converter is known to the personskilled in the art and makes use of a mode of controlling the boosttransistors 12, 22, . . . , 62 analogous to that used in the aboveembodiment for the buck transistors. Each of the individual boostconverters 100, 200, . . . , 600 constituted in this way can transferapproximately 250 W of power from the 14 V network to the 42 V network.

Advantageously, for a boost converter corresponding to FIG. 3,respective malfunction detectors D1, D2, . . . , D6 are connected viatwo inputs to the drain and the source of the boost transistor 12, 22, .. . , 62 of the individual converter with which it is associated. Thesemalfunction detectors D1, D2, . . . , D6 contribute in the same manneras described above to switching OFF the protection transistor 13, 23, .. . , 63 of the individual converter in which a malfunction is detected.

The same advantages and improvements as those mentioned for a buckconverter can be reproduced identically in the present case of a boostconverter.

FIG. 4 corresponds to a converter made up of reversible individualconverters 100, 200, . . . , 600. Each of the reversible individualconverters 100, 200, . . . , 600 uses the same components as the buck orboost chopper converters described above, and they are disposed inanalogous manner. A detailed description of these components is notrepeated below.

Each reversible individual converter 100, 200, . . . , 600 comprises abuck transistor 11, 21, . . . , 61 and a boost transistor 12, 222, . . ., 62 taking the places respectively of the diodes 11 a, 21 a, . . . , 61a and 12 a, 22 a, . . . , 62 a.

The buck transistor 11, 21, . . . , 61, e.g. an n-channel MOS-FET hasits drain connected to the terminal 1 and its source connected to thenode N1, N2, . . . , N6. Its intrinsic diode 18, 28, . . . , 68connected in parallel with the buck transistor 11, 21, . . . , 61 isoriented to pass current from the terminal 3 to the terminal 1.

The boost transistor 12, 22, . . . , 62, e.g. another n-channel MOS-FET,has its drain connected to the node N1, N2, . . . , N6 and its source tothe terminals 2 and 4. Its intrinsic diode 19, 29, . . . , 69 connectedin parallel therewith is oriented to pass current towards the node N1,N2, . . . , N6.

As shown in FIG. 5, for the reversible individual converter 100, itscontroller C1 possesses two outputs 110 and 120 connected respectivelyto the grid of the buck transistor 11, 21, . . . , 61 and to the grid ofthe boost transistor 12, 22, . . . , 62 of the individual converter 100.It also possesses two inputs connected by the wires 150 and 151 to thetwo terminals of the low-resistance resistor 15.

In a control mode known to the person skilled in the art, the controllerC1, when operating in a buck operating mode, controls the voltage at thegrid of the buck transistor 11, 21, . . . , 61 to switch it OFF and ONin alternation as a function of the value of the current measuredflowing through the low-resistance resistor 15. Simultaneously, itcontrols the grid of the boost transistor 12, 22, . . . , 62 so that itis OFF, at least during time intervals when the buck transistor 11, 21,. . . , 61 is ON.

Symmetrically, in a boost mode of operation, the controller C1 controlsthe grid of the boost transistor 12, 22, . . . , 62 to switch it OFF andON in alternation as a function of the value of the current measuredflowing through the low-resistance resistor 15. It then simultaneouslycontrols the grid of the buck transistor 11, 21, . . . , 61 so that itis OFF, at least during those time intervals during which the boosttransistor 12, 22, . . . , 62 is ON.

Similarly, each reversible individual converter 100, 200, . . . , 600includes a system for controlling its buck and boost transistors 11, 21,. . . , 61 and 12, 22, . . . , 62 identical with that of reversibleindividual converter 100. The control signals of all of the reversibleindividual converters are synchronized in identical manner to the buckor boost converters described above for the above embodiment.

In FIG. 4, the protection transistor 13, 23, . . . , 63 of eachreversible individual converter 100, 200, . . . , 600 is placed betweenthe inductor 14, 24, . . . , 64 and the low-resistance resistor 15, 25,. . . , 65, its drain being connected to the inductor, its source to theresistor, and its intrinsic diode 20 being oriented to pass current fromthe terminal 3 towards the terminal 1. These protection transistors 13,23, . . . , 63 connected in this way are controlled by the protectionmanagement unit P (see FIG. 5) itself associated with the malfunctiondetectors D1, D2, . . . , D6. These detectors D1, D2, . . . , D6 arerespectively connected to the source and to the drain of the bucktransistor 11, 21, . . . , 61 in each individual converter 100, 200, . .. , 600. The protection obtained is then identical to that of the firstembodiment, corresponding to FIGS. 1 and 2.

The same advantages and improvements as those mentioned for a buckconverter can be obtained in the present case of a reversible converter.

FIGS. 6 and 7 taken together correspond to a reversible converter ofstructure identical to that of the reversible converter shown in FIGS. 4and 5. In this new embodiment, the protection transistor 13, 23, . . . ,63 of each reversible individual converter 100, 200, . . . , 600 isdisposed between the terminal 1 and the buck transistor 11, 21, . . . ,61. Its drain is connected to the terminal 1 and its source to a nodebetween the drain of the buck transistor 11, 21, . . . , 61 and theconverter 16, 26, . . . , 66. The intrinsic diode 20, 30, . . . , 70 ofthe protection transistor 13, 23, . . . , 63 is still oriented to passcurrent towards the terminal 1.

This position for the protection transistor 13, 23, . . . , 63 ispreferred to a position situated between the buck transistor 11, 21, . .. , 61 and a node connecting the capacitor 16, 26, . . . , 66 to theterminal 1. The current flowing in the loop formed by the capacitor 16,26, . . . , 66, the buck transistor 11, 21, . . . , 61, and the boosttransistor 12, 22, . . . , 62 is a chopped current that is subject tosudden changes, so it is particularly advantageous to reduce thephysical size of this loop in order to reduce disturbances due to anyparasitic self-inductance in the loop, or indeed due to any radiationtransmitted from the loop.

The detector D1 still receives on its two inputs the voltage between thedrain and the source of the buck transistor 11 via the two wires 111 and112. An identical disposition is used for these components in each ofthe reversible individual converters 100, 200, . . . , 600.

The operation of the overall reversible converter in this embodiment,and the operation of its protection system, are identical to thecorresponding operations for FIGS. 4 and 5. Similarly, the detectorassociated with each buck transistor 11, 21, . . . , 61 enables theindividual converter 100, 200, . . . , 600 with which it is associatedto be taken out of service in the event of a short circuit occurring insaid buck transistor. The same improvements can likewise be combined inthis embodiment.

FIG. 8 shows a circuit which is further equipped with a protectiontransistor that is common to all of the cells, on the low-voltage side.

In this figure, the cells 100, . . . , 600 are represented bydashed-line boxes only, the individual structure of the cells being anyof the structure described above, with each cell incorporating a singleprotection transistor on its high voltage side.

In the FIG. 8 circuit, a high voltage DC network, e.g. operating atabout 42 V between terminals H1 and H2, includes a battery HR connectedbetween these terminals. H1 is a positive terminal and H2 is a negativeterminal.

A low-voltage DC network, e.g. operating at about 14 V between twoterminals B3 and B4 of this network B, includes a battery BR. Thebattery BR is connected between the terminals B3 and B4. B3 is apositive terminal and B4 is a negative terminal.

The low-voltage network is connected via a filter 800 to the cells 100,. . . , 600. The filter 800 is connected to the terminals 3 and 4 thatare common to the cells 100, . . . , 600. The structure of the filter800 is known to the person skilled in the art and is not described indetail herein.

Furthermore, the high-voltage network is also connected to the cells viaanother filter 700 connected to the terminals 1 and 2 and to theterminals H1, H2 of the high-voltage network having the high-voltagebattery HR connected between them.

A protection transistor 801, still an n-channel (N-MOS) metal-oxidesemiconductor field-effect transistor (MOS-FET) is connected between thefilter 800 and the terminal B3. The dragon of this protection transistor801 is connected to the filter 800, while its source is connected to theterminal B3.

A control unit CS has an output connected to the grid of the protectiontransistor 801 and an input connected to an output of a detector D. Thedetector D is also connected to the terminals B3 and B4.

In a normal mode of operation, the detector D detects a voltage of about14 V between the terminals B3 and B4. The control unit CS then causesthe safety transistor 801 to be switched ON by applying to its grid apositive voltage of about 5 V to 10 V relative to its source, forexample.

When the detector D detects abnormal values for the voltages B3 and B4,e.g. a polarity reversal, the control circuit CS interrupts the positivevoltage applied to the grid of the protection transistor 801. A resistor802, e.g. a 10 kΩ resistor connected between the grid and the source ofthe protection transistor 801 then ensures that this transistor switchesOFF. Thus, in the event of a short circuit or a polarity reversalbetween the terminals B3 and B4 of the low voltage circuit B, thelow-voltage circuit and the converter are isolated from each other.

When the protection transistor 801 is an n-channel MOS-FET, it possessesan external intrinsic diode 803 connected in parallel between the drainand the source of the transistor. This intrinsic diode 803 passescurrent from the source towards the drain of the transistor 801, with athreshold voltage of the order of 0.9 V to 1.3 V. The protectiontransistor 801 is oriented so that the intrinsic diode 803 passescurrent towards the filter 800.

Naturally, the invention is not limited to the embodiment described andvariants can be applied thereto without going beyond the ambit of theinvention as defined by the claims.

In particular, although the invention is described above for a preferredembodiment consisting in placing the single protection Transistor ofeach cell on the high-voltage side and the common switch on thelow-voltage side, thus enabling a cell to be isolated and alsoprotecting the converter against a polarity reversal on the low-voltageside while minimizing static consumption, it is possible to place theprotection transistors in each of the cells on the low-voltage side andthe common switch on the high-voltage side. At the cost of a smallincrease in static consumption, that makes it possible to avoidtransferring charge from the low-voltage battery to the high-voltagebattery when the high-voltage battery is discharged.

In embodiments that are alternatives to the embodiments described, theN-MOS type transistors may be replaced by corresponding transistors ofP-MOS type. They may also be replaced by transistors using bipolartechnology, without that changing the function and general operation ofthe circuit. The protection transistor which is common to all of thecells may also be replaced by a switch that is controlled by anelectromagnetic relay.

In addition, although the protection transistor 801 common to thevarious cells on the low-voltage side is shown as being integrated inthe converter, it may be located remotely therefrom.

Conversely, although the control unit CS for the transistor 801 is shownas being separate from the protection management unit P, it could beincorporated therein.

1. A DC/DC voltage converter comprising: a first positive terminal and afirst negative terminal for connection respectively to two terminals ofa high-voltage electrical network; a second positive terminal and asecond negative terminal for connection respectively to two terminals ofa low-voltage electrical network; and n cells connected in parallel,where n is an integer greater than unity, disposed between said firstpositive and negative terminals and between said second positive andnegative terminals, each cell comprising a chopper DC/DC converter, eachhaving a first circuit branch interconnecting said first and secondnegative terminals, a second circuit branch including an inductor andinterconnecting said first and second positive terminals, at least onechopper switch, and a first management unit adapted to control switchingof the chopper switch with a determined duty ratio; wherein each cellfurther comprises a single protection transistor disposed in said secondcircuit branch and associated with a protection management unit fortaking said cell out of service independently of the other cells;wherein the protection transistor of each cell is a MOS transistorconnected in series in said second circuit branch of the cell betweenthe inductor and said second positive terminal, and including anintrinsic diode having its cathode connected to the inductor and itsanode connected to said second positive terminal.
 2. A converteraccording to claim 1, wherein the single protection transistor in eachcell is connected in a high-voltage portion of the cell.
 3. A converteraccording to claim 2, wherein the MOS transistor connected in series insaid second circuit branch so as to be immediately adjacent to saidfirst positive terminal, and having the intrinsic diode connected tosaid first positive terminal by its cathode.